1. Field of the Invention
The present invention relates generally to a zoom optical system, and more particularly to a zoom optical system used on photographic cameras, especially lens shutter cameras.
2. Discussion of Related Art
Recently prevailing lens shutter cameras are generally used with a built-in zoom lens. In particular, cameras having a zoom ratio of 3 or greater are now in great demand.
A lens shutter camera is desired to be reduced in both size and weight even when the phototaking lens used therewith has a high zoom ratio. Thus, reductions in lens diameter, and in the total lens length upon the so-called collapsing operation where adjacent lens groups are housed in a camera body with a decreasing air space therebetween are an important challenge to the development of lens systems.
Various types of zoom lenses are known for lens shutter cameras, and for zoom lens systems having a zoom ratio of 3 or greater, a three-group type having a positive-positive-negative power profile is often proposed.
To achieve some wide-angle shots in such a three-group zoom system having a positive-positive-negative power profile with a zoom ratio of 3 or greater, it is required to increase the composite refracting power of the first and second lens groups, each having positive power, at the wide-angle end. In particular, it is required to increase the number of lenses in the second lens group for correction of off-axis aberrations at the second lens group.
To achieve satisfactory performance over a zooming space from the wide-angle to telephoto end, it is required to reduce the amount of aberrations at each zoom lens group. Especially, the third lens group should be made up of two or more lenses because there is an increase in the amount of aberrations produced at the third lens group at the telephoto end.
It is thus difficult to decrease the total lens length of the system upon collapsing because of the increasing number of lenses in the second, and third lens group and, hence, an increase in the axial lens total length of the system.
Solutions to these problems are proposed in publications, for instance, JP-A's 6-265787, 8-136809, 8-152559, 8-179215 and 8-262325.
In the system disclosed in JP-A's 6-265787, 8-136809, 8-152559, and 8-179215, however, a stop is located between adjacent lens groups. As a result, the lens total length increases upon collapsing and so makes the thickness of the camera body large although the axial thickness of each lens group is reduced. This is because it is required to allow for a sufficient space between adjacent lens groups, thereby preventing interference between the stop member and the lens upon collapsing.
In the system disclosed in JP-A 8-262325, on the other hand, the number of lenses in each lens group is decreased by effective use of an aspheric surface. Since the image circle diameter is smaller than those of other prior art systems, the axial lens total length of each lens group is increased and so the lens total length cannot be fully decreased upon collapsing. Again, the thickness of the camera body becomes large upon collapsing.